Organic electroluminescence (EL) devices are divided to into two types, i.e. a fluorescent type and a phosphorescent type. For each type, an optical device design has been studied according to the emission mechanism. For the phosphorescent organic EL device, it is known that due to its emission properties, a high-performance device cannot be obtained by simple application of the fluorescent device technique. The reason therefor is generally considered as follows.
The phosphorescent emission utilizes triplet excitons and thus uses a compound having a large energy gap in an emitting layer, since the energy gap value (hereinafter also referred to as singlet energy) of a compound is normally larger than the triplet energy value (referred to as the difference in energy between the lowest excited triplet state and the ground state in the invention) of the compound.
Therefore, in order to confine the triplet energy of a phosphorescent dopant material in an emitting layer efficiently, it is preferred that a host material having a larger triplet energy than a phosphorescent dopant material be used in the emitting layer. In addition, it is preferred that an electron-transporting layer and a hole-transporting layer be provided adjacent to the emitting layer, and a compound having a triplet energy larger than that of the phosphorescent dopant material be used in the electron-transporting layer and the hole-transporting layer.
As seen above, designing an organic EL device based on the traditional design concept leads to the use in the phosphorescent organic EL device a compound having a larger energy gap than that of a compound used in the fluorescent organic EL device, thereby to increase the driving voltage of the whole organic EL device.
In addition, a hydrocarbon-based compound having a high oxidation resistance and a high reduction resistance, which is useful for the fluorescent device, has a broad pi-electron cloud, and hence it has a small energy gap. Hence, for the phosphorescent organic EL device, such a hydrocarbon-based compound is unlikely to be selected, but an organic compound containing a hetero atom such as oxygen or nitrogen is rather selected. Consequently, the phosphorescent organic EL device has a problem that it has a shorter life as compared with the fluorescent organic EL device.
Further, the device performance is greatly affected by the fact that the relaxation rate of triplet excitons of a phosphorescent dopant material is very slower than that of singlet excitons thereof. That is, the emission from singlet excitons is expected to be efficient, since the rate of the relaxation leading to the emission is so rapid that excitons are unlikely to diffuse to the neighboring layers of an emitting layer (hole-transporting layer or electron-transporting layer, for example). On the other hand, since emission from triplet excitons is spin-forbidden and has a slow relaxation rate, the triplet excitons are likely to diffuse to the neighboring layers, so that the triplet excitons are thermally energy-deactivated unless the phosphorescent dopant material is a specific phosphorescent compound. In short, in the phosphorescent organic EL device, control of electrons and holes in the recombination region is more important as compared with the fluorescent organic EL device.
For the above reasons, enhancement of the performance of the phosphorescent organic EL device requires material selection and device design different from those of the fluorescent organic EL device.
Particularly, in the case of a phosphorescent organic EL device emitting blue light, it is preferred that a compound having a large triplet energy be used in an emitting layer and their neighboring layers as compared with a phosphorescent organic EL device emitting green to red light. Specifically, in order to obtain blue phosphorescent emission, it is ideal that a host material used in the emitting layer have a triplet energy of 3.0 eV or more. In order to obtain such materials, it has been required to design molecules according to a new concept which are different from those for materials for the fluorescent device and materials for the phosphorescent device emitting green to red light.
Under such conditions, as a material for a phosphorescent organic EL device emitting blue light, a compound having a structure in which plural heterocyclic rings are combined has been studied. For example, Patent Document 1 discloses a compound having a 3,3′-biscarbazole as a mother skeleton and a substituent which is adjacent to the carbon atom in each carbazole skeleton involved in the bond of the carbazoles. The document discloses the structure in which the biscarbazole is twisted by introducing an alkyl group to the substituent to retain the triplet energy high.
Patent Document 2 discloses a compound having a 3,3′-biscarbazole as a mother skeleton and a substituent which is adjacent to the carbon atom in each carbazole skeleton involved in the bond of the carbazoles. This document states advantageous effects that driving voltage is lowered and durability is improved are attained by using this compound as a host material of a phosphorescent device.